Conventionally, fins applied to flat tubes that have been used generally in a radiator for automobile and a condenser for car air conditioner are corrugated fins.
It seems that corrugated fins have almost reached to a saturated level technically and further contrivance to reduce air resistance significantly than the current level, to increase performance or to reduce weight is approaching to the limit.
Corrugated fins are applicable to radiators for automobiles, condensers for car air conditioners and outdoor units of air conditioners. However, there reside such problems as drainage of condensed water, frost accumulation in heating operation and the like. Therefore, the corrugated fins cannot be applied to heat exchangers (evaporators) for air conditioner indoor units, heat pump outdoor units or evaporators for refrigerators and automatic vending machines. Thus, the corrugated fins may be evaluated to be poor in applicability.
The reason of the above is as described below. That is, when incorporated into a heat exchanger, corrugated fins are remarkably poor in drainage performance due to the configuration thereof, and therefore frost gathers and grows swiftly; and further, the frost is hardly removed.
What can solve the above disadvantages of corrugated fins is plate fins having flat holes. However, being different from the case where a tube having a circular section (round tube) is used, it is difficult to insert the flat tube into the flat hole. To enhance insertion efficiency, clearance between the flat hole and the periphery of the flat tube has to be large enough. When the enough clearance is provided, the contact between the tube and the fin is deteriorated, resulting in reduction of performance.
Being different from the case of the round tube, the flat tube cannot be satisfactorily expanded from the inside thereof. To join the flat tube to the flat hole, brazing is necessary. However, when the clearance is formed large enough to ensure the insertion efficiency, brazing material is not satisfactorily supplied to the gap between the two; thus, the contact between the two is deteriorated, resulting in a reduction of the heat conductivity. Contrarily, when the clearance is formed small enough to ensure satisfactory brazing, the insertion efficiency of the tube is lowered, resulting in an extreme reduction of the productivity.
When the flat tube expanded on purpose is brazed, the final contact is satisfactorily ensured, even if the clearance is formed large enough. In such a case, however, expansion process of the tube needs to be added, which leads to such disadvantages as redundant process, poor productivity and a big investment for equipment.
A flat multi-hole tube is sometimes used for a heat exchanger of air conditioner. In this case, however, the tube is not allowed to be expanded.
So called an insertion type heat exchanger as described below has been proposed. That is, in place of forming flat holes in a long plate fin, slits of a U-like shape are formed from one end of a plate fin in the width direction thereof and flat tubes are forcibly inserted into the slits (refer to, for example, Patent Document 1).
According to this method, even when the clearance between the slit and the tube is small, the tube can be forcibly inserted easily into the fin from the side thereof. As a result, the contact between the tube and the fin is improved.
However, the plate type heat exchanger with slit fins as described above has a following disadvantage. That is, when the individual plate fins are gathered together and many slits are to be aligned, it is difficult to align them into a line properly and thus, handling is troublesome. Therefore, the plate type heat exchanger has not been put into volume production.
The following method is theoretically conceivable; i.e., many slits, which are parallel to each other in the width direction, are formed in a long plate fin, and flat tubes are pressed into the slits from one end thereof. However, it is difficult to align the slits in the plate fins and form a fin assembly practically.
Further, there arises such disadvantage that the core assembly has poor workability in engaging the tubes with the fins, since the slits are hardly aligned with each other. Accordingly, although the heat exchanger core of this type using flat tubes is theoretically recognized as an effective heat exchanger core, this heat exchanger core has not been put into practical use as far as the inventor of the present invention knows.
Accordingly, an object of the present invention is to provide plate fins for a heat exchanger and a heat exchanger core employing the same, that have good alignment efficiency for the slits of plate fins and thus excellent productivity of mass production.
Further, another object of the present invention is to provide plate fins for a heat exchanger and a heat exchanger core employing the same that allow the flat tubes to be engaged therewith in two rows.